A Michigan fisherman made an explosive catch Tuesday when he pulled up a World War I-era grenade from the depths of the Grand River.

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But when magnet fisherman Joseph Alexander first found the item, he had no idea what it was.

"I thought it looked like a grenade, but not one I've seen before," Alexander told WOOD-TV. "We brought it back home, took some pictures of it, posted it online, and we started getting comments that 'it's a grenade, call the police.' So that's what we did."

Police confirmed the find was a German Granatenwerfer from World War I. Authorities said because of the device’s age and length of time underwater, it was likely not dangerous. It will be safely stored until police explode it.

"They sent over an undercover vehicle and confirmed it was live, full, and they boxed it up, put it in a yellow bag, put it in the truck and took it out to what they called the field to detonate," Alexander told WOOD-TV.

Alexander has found some knives, but called this his first “cool find” since starting magnet fishing about a month ago.